Thread
Thread is a low-power wireless mesh protocol built on IEEE 802.15.4 radio (2.4 GHz, the same silicon as Zigbee) that speaks native IPv6. It is the network most battery-powered Matter devices use: sensors, buttons, and locks join a self-healing mesh and reach your home network through a Thread border router.
Thread solves the problem Wi-Fi cannot: multi-year battery life. An 802.15.4 radio draws a fraction of a Wi-Fi radio’s power, and the mesh topology means mains-powered Thread devices (plugs, bulbs) route traffic for battery ones, so coverage improves with every powered device you add. Unlike Zigbee, Thread carries IPv6 packets natively — a Thread device is addressable from your LAN like any other IP device, with no protocol translation, just routing.
Thread has no application layer of its own; it is purely transport. Matter provides the application layer on top, which is why the pairing badge usually reads “Matter over Thread.” Since Thread 1.3, border routers support standardized credential sharing, so border routers from different vendors (an Apple TV and a Nest Hub, say) can join the same Thread network instead of building parallel meshes — see the Thread border router entry for what that hardware does.